CHAPTER 1: INTRODUCTION
1.6ETHERNET BRIDGING
Bridging Overview
Ethernet bridges read in packets and make decisions to filter or forward based on the destination address of the packet. The simple filter/forward decision process allows a bridge to segment traffic between two networks, keeping local traffic local. This process increases the availability of each network while still allowing traffic destined for the opposite side of the bridge to pass.
A bridge connects two networks together and allows communications between the networks without the worry of distance violations or timing considerations between the two networks. Each individual network must be within maximum distance and timing specifications however. The bridge is considered to be a node on the network and stores and forwards packets on each network. Contrasted with a repeater that repeats the signal bit by bit from one side of the network to the other, the bridge actually reads each packet, checks the packet for accuracy, then makes a decision, based on the destination address, as to whether the packet should be sent to the other network. If the other network is busy, it is the bridge's responsibility to store the packet, for a reasonable time, until the transmission can be made.
It is also the responsibility of the bridge to handle collisions. If a collision happens as the bridge is transmitting onto the second network, the bridge is responsible for the back off and retransmission process. The original sending node is not made aware of the collision. It assumes the packet has been sent correctly. If for some reason the bridge is unable to send the packet to its final destination, the original sending station, expecting a response from the device it was attempting to contact, will "time out" and, depending on the protocol, attempt retransmission.
The bridge makes decisions on whether to forward or filter a packet based on the physical location of the destination device with respect to the source device. Bridges dynamically learn the physical location of devices by logging the source addresses of each packet and the bridge port the packet was received on in a table called the Source Address Table (SAT).
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